The disclosure of Japanese Patent Application No. 2000-151303 filed on May 23, 2001, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to an apparatus and a method for controlling a continuously variable transmission and, more particularly, to a technique for preventing deterioration of the durability caused by slippage of a power transmitting member.
2. Description of Related Art
For use in a continuously variable transmissions of a motor vehicle that transmits power or driving torque by use of friction with a power transmitting member, a control apparatus is known which is adapted to increase a clamping pressure applied to the power transmitting member when slippage of the power transmitting member is detected during a certain running period. An example of the control apparatus is disclosed in Japanese laid-open Patent Publication No. 9-324853. The continuously variable transmission disclosed in this publication is of a belt-and-pulley type having a pair of variable pulleys whose effective diameters are variable, and a torque transfer belt (serving as a power transmitting member) that is wound around the pulleys. In this transmission, the clamping pressure applied to the torque transfer belt is immediately increased if slippage of the torque transfer belt is detected while an engine load is increasing, for example, upon a start of the vehicle. Subsequently, if it is determined that the torque transfer belt is no longer slipping, the belt clamping pressure is returned to a normal clamping pressure that is determined based on the input torque and the speed ratio.
With the control apparatus of the continuously variable transmission as disclosed in JP-A-9-324853 as identified above, the clamping pressure is returned to the normal clamping pressure if the power transmitting member (i.e., the torque transfer belt) is no longer slipping, and therefore the power transmitting member is likely to slip again if the vehicle running or operating conditions become close to the conditions under which the previous belt slippage occurred, during running of the vehicle. This undesirably results in reductions of the service life or durability of the power transmitting member.
It is therefore an object of the invention to provide a control apparatus of a continuously variable transmission of a motor vehicle, which is adapted to suitably prevent a power transmitting member from slipping again after a slip was detected and eliminated by, for example, increasing a clamping pressure.
To accomplish the above and/or other objects, the invention provides an apparatus and a method for controlling a continuously variable transmission of a motor vehicle which is adapted to transmit power by use of friction with a power. transmitting member thereof. The apparatus includes a controller that: (a) detects slippage of the power transmitting member in a predetermined period of operation of the vehicle, (b) increases a clamping pressure that is applied to the power transmitting member when slippage of the power transmitting member is detected in the predetermined period; and (c) stores a state of increase of the clamping pressure in a memory. In the control apparatus, the controller increases the clamping pressure applied to the power transmitting member in accordance with a previous state of increase of the clamping pressure that is stored in the memory. With this arrangement, the power transmitting member is prevented from slipping again during a current period of operation of the vehicle following the previous operating period in which slippage of the power transmitting member occurred. It is thus possible to avoid a reduction in the durability of the power transmitting member due to slipping thereof.